Pressure control mechanism for doctor blades



Jan. 12, 1960 F. FUCHS PRESSURE CONTROL MECHANISM FOR DOCTOR BLADESFiled March 13, 1958 3 Sheets-Sheet l INVENTOR Jan. 12, 1960 F. FUCHS2,920,557

PRESSURE CONTROL MECHANISM FOR DOCTOR BLADES Filed March 13, 1958 sSheets-Sheet 2 Fundy-AL Fua. A 5

BY 7 MW 6/ A oRrvEYs 1N VENTOR Jan. 12, .1960 F. FUCHS 2,920,557

PRESSURE CONTROL MECHANISM FOR DOCTOR BLADES Filed March 13, 1958 3Sheets-Sheet 3 1 N VENTOR United States Patent PRESSURE CONTROLMECHANISM FOR DOCTOR BLADES Friedrich Fuchs, Ende, uber Dortmund,Germany, assignor to Westfalendruck G.m.b.H. & Co. Kommandit-Gesellschaft, Dortmund, Germany, a firm of Germany Application March 13,1958, Serial No. 721,246

Claims priority, application Germany March 21, 1957 7 Claims. (Cl.101-157) The present invention relates to improvements in rotogravureand similar printing presses and more particularly to a new method and anew device for uniformly eliminating the ink from the unetched surfaceof a printing cylinder by means of a doctor blade.

in rotogravure and similar printing methods in which the image to beprinted is engraved or etched into the printing cylinder, it iscustomary to dip the surface of the cylinder into a pool of ink or toapply the ink upon the surface of the printing cylinder by means of aninking roll, and then to remove the surplus ink from the printingcylinder by means of a doctor blade. During the printing process, theink remaining within the engraving or etching in the cylinder surfacewill then be taken up by a web of paper, cloth, or the like which ispressed by an impression roll against the printing cylinder.

Apart from removing the surplus ink from the printing cylinder it isalso the purpose of the doctor blade to control very accurately theamount of ink which should be allowed to pass between the edge of thedoctor blade and the surface of the printing cylinder at the engraved oretched points of the latter so that the printing on the respectiveproducts will always be uniform.

Although the various known doctor-blade designs differ from each otherinsofar as their particular construction is concerned, most of themareequal in principle and all have certain disadvantages which preventthem from accomplishing their purpose completely. Although many efiortshave been made for a long time to overcome these disadvantages, theywere all inadequate because their proper function depended upon thesensitiveness of touch of the persons operating the same so that animproper operation or inaccurate adjustments could never be entirelyavoided.

Thus, for example, the following suggestions have been made as to howthe amount of ink which should be permitted to pass between the doctorblade and the etched surface of the printing cylinder could be made moreuniform so that the printed products of each series would all be of anequal quality or appearance.

in a doctor blade system which is operated by a crank, the uneven inkelimination or ink passage is to be rectified by increasing the bladepressure upon the surface of the printing cylinder at the time of achange in the direction of movement of the doctor blade, that is, whenthe blade is at a stop, and to such an extent that, when the blade isstanding still, the amount of ink passing between the blade and thecylinder surface will be the same as at the time when the crank isturned at an angle of 90, that is, when the blade moves at its maximumspeed. During the movement from the dead-center position to the 90position, the pressure of the blade is changed in proportion to theacceleration thereof so that the amount of ink passing between the bladeand the cylinder surface will become more uniform.

The principle of compensating the ink passage at the time of thestandstill by increasing the blade pressure "ice 2 has also been appliedto doctor blade systems in which the sliding reciprocatory movement ofthe blade is positively controlled by cams.

Another prior attempt to solve the difiiculty of regulating the inksupply on the printing cylinder consisted in the provision of a doctorblade construction in which a plurality of closely adjacent resilientelements, which to some extent are independent of each other, exert apressure upon the blade so as to hold it in a resilient engagement withthe cylinder surface. These resilient elements are formed by a steelblade in a shape similar to that of the doctor blade which is serratedalong one edge so as to form a plurality of free spring fingers. Thesteel blade itself is clamped firmly along its entire length and it isdisposed at an angle to the doctor blade so that the tips of the springfingers rest upon the doctor blade and are given a spring tension byflexing under the pressure applied upon the steel blade in a directiontoward the doctor blade. However, since the individual spring fingers ofthis construction are only resilient by flexing at an angle to theirnormal direction, and since they are combined with each other by acommon rigid back portion, this prior device is not capable of solvingthe difficulties previously described.

It is an object of the present invention to provide a doctor blade and acontrol mechanism therefor which is designed to regulate the pressure ofthe doctor blade upon the surface of the printing cylinder so that thispressure will adapt itself automatically at any point along the lengthof the doctor blade to any irregularities either of the cylinder surfaceor of the doctor blade itself so that an exactly uniform amount of inkwill be allowed to pass at all times between the cylinder surface andthe doctor blade and the printing will always be uniform throughout eachseries of products to be printed.

According to the present invention this object will be attained byproviding a resilient element which acts upon the doctor blade andautomatically adjusts and compensates any changes in tension which mightoccur during the printing operation at any point between the doctorblade and the surface of the printing cylinder after the doctor bladehas been adjusted in the conventional manner with respect to theprinting cylinder. The resilient element according to the invention isdevised so as to adjust itself to permit any irregularities which mightoccur along the length of the printing cylinder or of the doctor bladeto be compensated individually and separately from each other.

The resilient element according to the present invention may be ofvarious designs. Thus, for example, it may consist of a plurality ofseparate pressure members which are mounted in a row adjacent to eachother and each of which is devised so as to exert a resilient pressureupon the doctor blade entirely independently of the other members. Suchresiliency may be attained by subjecting each pressure member to theaction of a separate spring or the like, or by subjecting all pressuremembers to the action of a common resilient cushion element. Suchcushion may, for example, consist of a resilient tube or the like whichis filled with air or a fluid, the pressure of which may be adjusted bysuitable control means. Such device permits a uniform pressure to beexerted upon the doctor blade and will thus insure that uniform amountof ink remains on the etched or engraved surface of the printingcylinder as long as the basic adjustment of the doctor blade is notchanged. However, such an adjustment is sometimes necessary with theresult that, because of the change in position of the pressure memberswithin their common housing, the pressure ratio and thus also thespecific pressure of the individual pressure members upon the doctorblade will change. In such cases, it will therefore be necessary tocarry out a new adjustment. If the individual pressure members are actedupon by springs, there is the further disadvantage that the adjustmentdepends upon the sensitiveness of touch of the operator and that it isthus not always possible to insure an absolutely uniform pressure of allof the pressure members upon the doctor blade. Consequently, it is thennot always possible to control the passage of ink as accurately as maybe required to attain uniform printing results.

It is another object of the invention to overcome these disadvantages bythe provision of means whereby a change in position of the pressuremembers within their common housing will not affect the specificpressure of each member upon the doctor blade, and wherein theadjustment will not depend upon the sense of touch of the operator.These means according to the invention have the further advantage ofincreasing the reactivity of each individual pressure member todifferences in the ink passage.

Thus, according to a further embodiment of the invention, eachindividual pressure member may consist of a cylinder which extends atone end into a common pressure chamber and of a piston within eachcylinder which is adapted to reciprocate therein. This piston issubjected at one end to the pressure within the pressure chamber andacts at the other end upon the doctor blade. The piston which is made ofa rod of a diameter smaller than the inner diameter of the cylinder issuspended within the cylinder by means of a thin rubber tube, one end ofwhich may be closed and which is slipped with this end portiondownwardly over the upper end portion of the piston rod so that this endportion of the tube will cling firmly to the end portion of the pistonrod, whereupon the free end of the tube is inverted upon itself toextend upwardly along the inner wall of the cylinder and beyond itsupper edge, where this extending end is again inverted and foldeddownwardly over this upper edge of the cylinder so that the open endportion of the tube surrounds and clings to the upper part of thecylinder. Upon a reciprocating movement of the piston within thecylinder, the rubber tube will then roll along the inner wall of thecylinder and the outer wall of the piston rod by unrolling from one androlling upon the other. In this manner, the upper end of the cylinderwill be hermetically closed at all times relative to the common pressurechamber, and the piston surface which is acted upon by the pressurewithin the pressure chamber through the interposed rubber tube willalways remain constant so that the specific pressure of the piston uponthe doctor blade will also always remain constant.

Since the piston is made in the form of a rod of a certain length, it ispreferably provided with collars or the like which are adapted to slidealong the inner wall of the cylinder so as to guide the piston without,however, exerting any compression or suction during the reciproeatingmovement. The lower end of the rodlike piston which projects from thecylinder preferably carries a small pressure plate which is adapted topress upon the doctor blade and is connected to the piston so as to bepivotable about an axis transverse to the longitudinal direction of theprinting cylinder. The individual pressure plates on the lower ends ofthe pistons are disposed in a row along the doctor blade and preferablyextend substantially vertically thereto. They are spaced at a relativelysmall distance from each other which, however, is suflicient to alloweach pressure plate to pivot to some extent relative to the adjacentplates.

Further objects, features, and advantages of the present invention willbe apparent from the following detailed description thereof,particularly when read with reference to the accompanying drawings inwhich- Fig. 1 shows a cross section of the doctor blade arrangement andthe new pressure control mechanism therefor, taken along line I--I inFig. 2;

Fig. 2 shows a cross section taken along line II--II in Fig. 1;

Fig. 3 shows a modification of the pressure control mechanism accordingto the invention in cross section similar to Fig. 1 but on an enlargedscale;

Fig. 4 shows the pressure control mechanism according to Fig. 3 in adifferent position of its operation;

Fig. 5 shows a side view of the lower part of Fig. 4, and

Fig. 6 is an enlarged cross-section through the central part of themechanism of Fig. 3.

Referring to the drawings, and first particularly to Figs. 1 and 2thereof, a shaft 1 carries a clamp support 2 which is adjustably securedto shaft 1 by clamping bolts 3 and, in turn, firmly clamps the doctorblade 4. At both sides of clamping support 2, arms 5 and 6 are securedto shaft 1 and support the pressure control mechanism according to theinvention. This mechanism consists primarily of a substantiallycylindrical tube 7 with a projecting longitudinal flange 8 thereon whichis provided with a slot extending from one end of the tube to the other.A large number of pressure members 9 having enlarged heads or endflanges integral therewith are slipped from one end of tube 7 into thisslot so as to be disposed therein in a row adjacent to each other. Thethin ends of pressure members 9 project from flange 8 of tube 7 and arespaced only a small distance from each other. The lower beveled ends ofpressure members 9 together form a perfectly straight serrated bladelikeedge, with the exception of a small number of pressure members 9 at eachend of tube 7 which are made of a shorter length for a reason statedbelow. The length of the entire row of pressure members 9 and 9' issubstantially equal to the length of the doctor blade 4 which is securedin clamp support 2. For maintaining the pressure members 9 and 9 in afixed position in the longitudinal direction of tube 7, end stops 10 aresecured in both ends of the slot in flange 8. Tube 7 is further providedat both ends with screw threads upon which end caps 11 and 12 aretightly screwed. The closed chamber formed within tube 7 contains aflexible tube 13, for example, of rubber. A check valve 14 is connectedto rubber tube 13 and leads through a pipe or hose, not shown, toapressure tank or the like for supplying a suitable pressure medium, forexample, compressed air, to tube 7. Suitable control means forregulating such pressure may also be provided. End caps 11 and 12terminate in short shaft stubs 15 which are rotatably mounted in sockets16 at the outer ends of arms 5 and 6. In order to permit the entirepressure control unit consisting of the outer tube 7, the inner tube 13and end caps 11 and 12 to be easily inserted into or removed fromsockets 16, the latter are provided with slots 17 while the shaft stubs15 are provided with flat parallel surfaces 18 which are spaced apart ata distance substantially equal to the width of slots 17 so as to permitshaft stubs 15 to slide into or out of sockets 16 through slots 17.After the entire unit 7, 11, 12, 13 is thus inserted into sockets 16, itmay be turned by means of a handle 19 about an angle of about to theposition as indicated in Fig. 1, whereby surfaces 18 will be likewiseturned relative to slots 17 so that shaft stubs 15 will then be securelyretained in sockets 16.

In accordance with the thickness and flexibility of the respectivedoctor blade 4 used, it is necessary to adjust the distance of thepoints of contact between the lower ends of pressure members 9 anddoctor blade 4 from the edge of the blade which is adapted to engagewith the surface of printing cylinder 20. For this purpose, a stopmember 21 is secured to end cap 11. This stop member 21 is adapted toengage with one end of an adjusting screw 22 which is screwed into thehead 23 of another screw which is screwed into arm 5. A lock nut 24 maybe used to lock the adjusting screw 22 in its adjusted position. Afterthe pressure control unit has been inserted through slots 17 and turnedby handle 19 until stop member 21 engages with the properly setadjusting screw 22, a screw on socket 16 on arm 6 may be tightened tolock the entire unit in such a position.

The adjustment and operation of the new pressure control mechanism asillustrated in Figs. 1 and 2 is as follows:

After the clamp support 2 with the doctor blade 4 securely clampedtherein has been lightly tightened on shaft 1 by means of clamping bolts3, clamp support 2 is adjusted by means of setscrews 27 relative to thesurface of printing cylinder 20 so as to extend parallel thereto,whereupon clamping bolts 3 are firmly tightened. Rubber tube 13 is thenfilled with a gaseous or fluid elastic medium, for example, compressedair, so as to contain a certain pressure which should be slightly lowerthan the pressure which would be required to eliminate the ink entirelyfrom the surface of printing cylinder 20. This pressure may be veryaccurately adjusted by means of a precision pressure gauge, not shown,which is connected to the pressure line leading to tube 13. Rubber tube13 will thus press the end flanges 26 of pressure members 9 upon theirseats in tube 7 so that the thin lower ends of pressure members 9 willuniformly engage with and resiliently press upon doctor blade 4.

By means of a conventional tensioning lever, not shown, doctor blade 4is then placed in the operative position and properly adjusted relativeto printing cylinder 20 in the following manner:

By turning handle 19, the resilient pressure members 9 are moved to suchan extent toward the edge of doctor blade 4 that the blade can no longerbe bent so as to change its tangent angle relative to cylinder 20.Thereupon, the rotation of printing cylinder 20, which passes in theusual manner through a pan of ink, is started in the direction shown bythe arrow in Fig. l, and the edge of blade 4 is pressed against thesurface of cylinder 20 until the shorter pressure members 9' at bothends of tube 7 will engage with doctor blade 4 and pressure members 9will be forced for a certain distance back into tube 7. It will thenusually be found that the minimum pressure which is at first maintainedin the resilient inner tube 13 will not be sufiicient to remove the inkentirely from the surface of cylinder 20. The pressure in tube 13 isthen gradually increased until pressure members 9 press so strongly upondoctor blade 4 that the cylinder surface will be wiped entirely clean ofink so that the printing operation may then be started.

The proper pressure required for the respective printing form may thenbe determined on the mentioned precision gauge and its amount may serveas a basis for the adjustment for subsequent printing operations. Thispressure within tube 13 and thus also the pressure of members 9 and 9'upon the doctor blade may then be maintained at a constant value by asuitable automatic pressure regulator, not shown.

The modification of the invention as illustrated in Figures 3 to 5 is inprinciple similar to the embodiment as shown and described above inFigs. 1 and 2. A tubular member is mounted on the shaft of the doctorblade in a manner similar to that shown in Figs. 1 and 2. Tube 30 islikewise slotted longitudinally, and a bearing member 31 correspondingto flange 8 in Fig. 1 and bridging the slot is tightly secured to thefree end portions 32 of tube 30 so that the inside 'of tube 30 will forma hermetically closed pressure chamber. Bearing member 31 extendssubstantially along the entire length of tube 30 and supports aplurality of cylinders 33 which are disposed in a row closely adjacentto each other and are tightly sealed within member 31. For this purpose,bearing member 31 has a plurality of stepped bores, the narrower innerpart 34 of which has a diameter corresponding to the outer diameter ofeach cylinder 33, while the wider outer part 35 is adapted to receive aflange 36 on the lower end of each cylinder 33 and is provided Withinner screw threads 37 into which a screw plug 38 is screwed to pressflange 36 firmly upon a gqasket 39 and thus seal the outer wall of thecylinder hermetically toward theoutside. A rodshapedpiston 40 isslidably mounted within each cylinder 33 and guided therein for movementin a direction coaxial with the cylinder by means of a pair of collars41 or the like which, however, do not produce any compression or suctionof air within cylinder 33 and may for this purpose be provided, forexample, with one or more axially extending grooves, not shown, in theirouter surface. In order to prevent piston 40 from turning about itslongitudinal axis, the lower part 42 thereof may be of a rectangularcross section and be guided by the walls of a rectangular opening in theend flange 36 of cylinder 33. The lower reduced end 43 of piston 40 isslotted and carries within such slot a thin pressure plate 44 which issecured to piston 40 by a screw 45' so as to be freely pivotable to acertain extent about the axis of this screw. Thus, pressure plate 44will be capable of adapting itself to any small unevenness on thesurface of doctor blade 46.

The open upper end of each cylinder 33 is hermetically closed by meansof a thin rubber tube 47, one end of which isdrawn tightly over theupper end 48 of piston 40 and may, if necessary, be secured thereto nearthe upper end to prevent piston 40 from pulling out of this tube duringits downward movement. Rubber tube 47, after being drawn downwardly overthe upper end 48 of piston 40 is inverted upon itself so as then toextend upwardly along the inner wall of cylinder 33, and is then againinverted at the upper edge of cylinder 33 and drawn downwardly so thatits other end portion clings tightly to the outer surface of cylinder33. Apart from hermetically closing the inside of the common pressurechamber 3% toward cylinder 33 and piston 40, rubber tube 47 serves as ameans of suspending piston 40 on cylinder 33 and especially insures thatupon an axial movement of piston 40 relative to chamber 36 or upon a newadjustment of chamber 30 in the longitudinal direction of cylinders 33,the active piston surface which is acted upon by the pressure withinchamber 30 will always remain of equal size since tube 47 whileunrolling either from the outer wall of piston 40 or from the inner wallof cylinder 33 automatically rolls itself upon the other wall.

This may be more clearly understood from a comparison of Figs. 3 and 4which show that the active piston surface b always remains of the samesize and equal to the'circular area formed by the inner diameter of thecylinder minus the thickness of the wall of rubber tube 47, even thoughthe distance a between the lower end of the cylinder and the surface ofthe doctor blade 46 might change considerably. Consequently, thespecific pressure of piston 40 or of pressure plate 44 will also alwaysremain the same.

The results and advantages attained by the present invention may besummarized as follows:

Since the pressure of the pressure members 9 or 44 is exerted upondoctor blade 4 or 46 at points which are closely adjacent to the veryedge of the blade, the latter maybe made of very thin strip steel whichis capable of entering into the smallest depressions in the surface ofthe printing cylinder and eliminating the ink therefrom completelyWithout requiring any increase in the blade pressure over that requiredfor an absolutely even surface of the cylinder as was previouslynecessary with other doctor blade arrangements. The material of thedoctor blade may now, in fact, be made so thin that its edge no longerneeds to be beveled, trimmed, or trued. Even a slightly wavy shape ofthe blade will not have any disadvantageous efiect since the individualpressure members will engage in the waves without changing the pressureupon the blade or that exerted by the blade edge upon the surface of theprinting cylinder.

The doctor blades need also no longer be given any initial tension sincethe resilient pressure members which act directly upon the doctor bladeclosely adjacent to the blade edge will compensate any differences intension and will insure that the pressure of the blade edge upon thecylinder surface will remain entirely uniform. For this reason, anydeformation of the blade due to uneven wear has also no effect upon theelimination of ink from the cylinder surface or the passage of inkbetween the etched cylinder surface and the doctor blade.

The effect of the back pressure underneath the doctor blade may also beeasily overcome by increasing the pressure within the pressure chamber13 or accordingly. This may be done automatically by suitablepressureregulating means of a known design which are responsive to theoperation of the driving mechanism of the machine.

Rhythmical vibrations within the doctor blade can no longer occur sincethe impacts upon the blade which are caused by the etched portions onthe cylinder will be compensated by the new pressure control mechanism.

Changes in the position of the doctor.blade on the cylinder surface dueto inaccurate grinding means will also not affect the ink passage or inkelimination since the resilient pressure members follow any movement ofthe doctor blade without changing the pressure thereon.

The pressure members and thus also the doctor blade will also follow thecylinder surface if the latter is running out of true and without anyincrease or decrease in pressure of the blade upon the cylinder surface.Consequently, the ink will be uniformly eliminated from the cylindersurface even at such times so that the printed products will be of equalappearance and quality.

The grooves which by previous doctor blade arrangements were often cutinto the edge portions of the printing cylinder will also no longeroccur since the doctor blade is no longer pressed against the cylindersurface by a tensioning lever or the like, but the pressure is nowproduced indirectly by the resilient pressure members acting upon thedoctor blade. The ends of the doctor blade which in the reciprocatorymovement thereof will alternately be free of the cylinder surface are,according to the invention, not under any tension and will be depressedby the shorter pressure members 9' out of the straight line of the bladeedge only by a few tenths of a millimeter.

For increasing the blade pressure, it is also no longer necessary tochange the tangent angle between the doctor blade and the cylindersurface since the pressure increase is attained by one within thepressure chamber of the new control mechanism.

In comparison to previous doctor blade arrangements, the new controlmechanism has the further advantage that the amount of pressure of thedoctor blade upon the surface of the printing cylinder can be accuratelymeasured and controlled, and can therefore be uniformly maintained atall times.

Although my invention has been illustrated and described with referenceto the preferred embodiments thereof, I wish to have it understood thatit is in no way limited to-the details of such embodiment, but iscapable of numerous modifications within the scope of the appendedclaims.

Having thus fully disclosed my invention, what I claim 1. In combinationwith a doctor blade for a printing press having a printing cylinder, aplurality of adjacent pressure members disposed in a row and adapted toengage resiliently with said doctor blade along the length thereof formaintaining said blade in resilient engagement with the surface of saidprinting cylinder, means for mounting said pressure members so as to bemovable independently of each other in a substantially verticaldirection relative to said doctor blade, common resilient pressure meansadapted to act simultaneously upon all of said pressure members, meansfor adjusting and controlling the resilient pressure exerted by saidpressure means, said common pressure means including a tubular housinghaving closed ends containing a fluid pressure medium, and meansengaging said pressure members for applying the pressure in the cylinderthereto, said doctor blade being mounted on a shaft, a pair ofsupporting arms secured to said shaft near both ends of said doctorblade, and means on the two closed ends of said housing for removablysecuring said housing to said supporting arms.

2. In combination with a doctor blade for a printing press having aprinting cylinder, a rigid tubular housing, a plurality of pressuremembers disposed in a straight row closely adjacent to each other andadapted to engage resiliently with said doctor blade along the lengththereof, each of said pressure members having an upper end disposedwithin said housing and a lower end projecting from said housing andadapted to engage with said doctor blade, and being slidably guidedwithin an aperture in the wall of said housing for movementindependently of the other pressure members in the longitudinaldirection of said member and substantially vertically to said doctorblade, a closed elastic rubber tube within and enclosed by the wall ofsaid housing and having a longitudinal periphery adjacent the row ofpressure members adapted to exert a resilient pressure upon the upperends of all of said pressure members, and means for filling said rubbertube with a pressure fluid and for adjusting and controlling thepressure of said fluid within said rubber tube.

3. In a combination as defined in claim 2, wherein said aperture isformed by a slot in the wall of said housing extending longitudinally ina straight direction from one end of said housing to the other, all ofsaid pressure members being inserted into said slot from one end of saidhousing so as to form a straight row, and being guided by the walls ofsaid slot.

4. In combination with a doctor blade for a printing press having aprinting cylinder, a rigid tubular container forming a pressure chamber,said container having a plu rality of closely adjacent apertures in itswall disposed in a straight row extending longitudinally from a pointnear one end of said container to a point near the other end, a cylinderhaving open ends mounted in each of said apertures and extending intosaid container and parallel to the other cylinders, a piston slidablewithin each of said cylinders in the axial direction thereof, andsubstantially vertical to said doctor blade, the lower end of saidpiston projecting from said cylinder and adapted to engage resilientlywith said doctor blade, at least the upper end portion of said pistonhaving a diameter smaller than the inner diameter of said cylinder, athin rubber tube connecting said upper end portion of said piston withthe upper end portion of said cylinder within said container forsuspending said piston within said cylinder and for hermetically sealingsaid pressure chamber relative to said piston and the inside of saidcylinder, and means for supplying a pressure fluid to said pressurechamber and for adjusting and controlling the pressure within saidpressure chamber, said pressure within said chamber being adapted to actupon said rubber tubes on all of said cylinders and pistons to presssaid pistons resiliently upon said doctor blade.

5. A combination as defined in claim 4, wherein one end of each of saidrubber tubes is drawn for a certain distance downwardly over the upperend of one of said pistons and said rubber tube is then inverted toextend upwardly along the inner wall of said cylinder and is then againinverted to extend downwardly over the upper edge of said cylinder sothat the other end of said rubber tube tightly encloses the upper endportion of said cylinder.

6. A combination as defined in claim 4, wherein the lower end of each ofsaid pistons is slotted, and further comprising a small pressure platepivotably mounted Within said slot so that its lower edge when inengagement 2,920,557 9 10 with the surface of said printing cylinder canadapt itself References Cited in the file of this patent to anyirregularity in the shape of said surface.

7. A combination as defined in claim 6, wherein said UNITED STATESPATENTS piston is guided within said cylinder so as to be non- Owen Nov.24, 1925 rotatable about its longitudinal axis. 5 Leech July 7, 1931

